Hello there! Yes, it does work - HURRAH! Let me tell you how!
Well, one reason it didn't work was that I was using diaries instead of notebooks, really cheap ones, and they were ultra thin - supermarket-style thin. Mistake no.1. Once you'd finished you couldn't put anything big in! I wanted my felt cover to be used again and again and again NOT for one year then throw it in the bin! Anyway, browsing the internet, as you do, I saw some notebooks, very thick ones. I really liked them so I researched "felted notebooks" and found some....lots of them!
Greatly inspired, I did some more research and found a site which sold THICK notebooks (AND some 4 x 6" ones with "olde worlde" crinkled edges - yes yes yes!) And I bought them.
On pen and paper I worked out the measurements. At one point I even considered notebooks which were stuck to the felt ! I got my glue gun out and smeared glue all over the front cover, stuck the felt on...and waited....and waited....and I tentatively tried lifting a corner...and the whole thing came away!! It hadn't worked. Mistake no.2. So I began thinking of my notebook as an interchangeable thing. I found some machine felt
to cover the inside edges and sewed it on...and it was too large. Mistake no.3...so I got my scissors out and chopped off the excess, pulled the machine-made felt off and cut that to the new size (had to hand stitch it on because it was a really tight fit!) and, hey presto. I fitted the notebook inside the felt - wrestled with it slightly because, as I said, the whole thing was a bit tight, it being my first book and all - and it worked!
A bit later on (with a few more cuts, scrapes and bad language!) I'd mastered it. Even tried making a patchwork felt cover out of off-cuts and that worked as well! The pics I'm showing are just the beginning of the felt story but it will give you a flavour...